Steel material for taylor welded blank and method for manufacturing hot-stamped part using same steel

ABSTRACT

In accordance with one aspect of the present disclosure, there is provided a steel material for a tailor-welded blank, including 0.04 to 0.06 wt % of carbon (C), 1.2 to 1.5 wt % of manganese (Mn), 0.01 to 0.10 wt % of titanium (Ti), 0.01 to 0.10 wt % of niobium (Nb), and the balance of iron (Fe) and inevitable impurities; having a tensile strength (TS) of 550 MPa or greater, a yield strength (YS) of 300 MPa or greater, and an elongation (EL) of 20% or greater; and having a dual-phase structure of ferrite and martensite.

TECHNICAL FIELD

The present disclosure relates to a steel material for a tailor-weldedblank and a method of manufacturing a hot-stamped part using the same,and more particularly, to a steel material for a tailor-welded blank,which has improved elongation and performance as an shock absorbingmaterial while minimizing the variation in properties thereof, whichdepends on hot-stamping process parameters, and a method ofmanufacturing a hot-stamped part using the same.

BACKGROUND ART

In recent years, the automobile industry has demanded strict automotivecrash performance to enhance passenger safety. In addition, as theawareness of the environment has raised, the fuel economy standards forexhaust gas regulations have been strengthened, and thus the need forautomobile body weight reduction has continuously increased. As part ofefforts to simultaneously satisfy the demands of improving the crashperformance and reducing the automobile body weight, the application ofhigh-strength steel plates to automobile bodies has continuouslyincreased. In manufacturing the automobile body, high-strength parts areapplied to reinforce protection against side crashes, because they playa very important role in securing a survival space for the driver when aside crash occurs. High-strength steel material corresponding to a classof 150K, which is mainly used as an automobile crash energy absorbingmember, undergoes brittle fracture which threatens the safety of thedriver when a side crash occurs. For this reason, another member isconnected to the lower end of the high-strength steel material, whichundergoes brittle fracture, by a tailor-welded blank (TWB) process,thereby increasing the crash energy absorption ability of thehigh-strength steel material.

The prior arts related to the present disclosure include Korean PatentApplication Laid-Open Publication No. 2016-0061560 (published on Jun. 1,2016; entitled “Method for Manufacturing Taylor Welded Blank”).

SUMMARY OF THE INVENTION Technical Problem

A problem to be solved by the present disclosure is to provide ahot-stamped steel material, which may have improved elongation and crashperformance while minimizing the variation in properties thereof bycontrolling alloying elements and process conditions, and a method ofmanufacturing a hot-stamped part using the same.

Technical Solution

In accordance with one aspect of the present disclosure, provided is asteel material for a tailor-welded blank, including 0.04 to 0.06 wt % ofcarbon (C), 1.2 to 1.5 wt % of manganese (Mn), 0.01 to 0.10 wt % oftitanium (Ti), 0.01 to 0.10 wt % of niobium (Nb), and the balance ofiron (Fe) and inevitable impurities.

In the present disclosure, the steel material for a tailor-welded bankmay be a steel material having a tensile strength (TS) of 550 MPa orgreater, a yield strength (YS) of 300 MPa or greater and an elongation(EL) of 20% or greater, and having a dual-phase structure of ferrite andmartensite.

In the present disclosure, the steel material for a tailor-welded blankmay further include more than 0 wt % and not more than 0.03 wt % ofsilicon (Si), more than 0 wt % and not more than 0.018 wt % ofphosphorus (P), and more than 0 wt % and not more than 0.003 wt % ofsulfur (S).

In the present disclosure, the steel material for a tailor-welded blankmay further include an aluminum (Al)-silicon (Si) plating layer forimproving corrosion resistance on the surface of the steel material.

In accordance with another aspect of the present disclosure, provided isa method for manufacturing a hot-stamped part, including the steps of:preparing a first blank using a steel slab including 0.04 to 0.06 wt %of carbon (C), 1.2 to 1.5 wt % of manganese (Mn), 0.01 to 0.10 wt % oftitanium (Ti), 0.01 to 0.10 wt % of niobium (Nb), and the balance ofiron (Fe) and inevitable impurities, and a second blank obtained bycutting a steel plate provided separately from the first blank; forminga joined steel plate by welding the first and second blanks to eachother by a tailor-welded blank process; forming a molded body byhot-stamping the joined steel material in a press mold; and forming ahot-stamped part by cooling the molded body.

In the present disclosure, the step of preparing the first blank mayinclude the steps of: finish-hot-rolling the steel slab at a finishingdelivery temperature (FDT) of 860° C. to 920° C.; cooling thefinish-hot-rolled steel plate to a coiling temperature (CT) of 620° C.to 660° C., followed by coiling; uncoiling the coiled steel plate,followed by cold rolling; and subjecting the cold-rolled steel plate toannealing heat treatment.

In the present disclosure, the second blank may be formed into a steelplate having a tensile strength of 1,200 to 1,500 MPa.

In the present disclosure, the step of forming the molded body mayinclude the steps of: heating the joined steel plate at a temperature of850° C. to 950° C.; and transferring the heated joined steel plate tothe press mold within a transfer time of 9 to 11 seconds.

In the present disclosure, the cooling of the molded body in the step offorming the hot-stamped part may be performed at a rate of 30 to 120°C./s.

In the present disclosure, the first blank may function as a shockabsorbing element for an automotive B-pillar, and the second blank mayfunction as a crash support element for the automotive B-pillar.

In the present disclosure, the first blank may have a tensile strength(TS) of 550 MPa or greater, a yield strength (YS) of 300 MPa or greater,and an elongation (EL) of 20% or greater, and have a dual-phasestructure of ferrite and martensite, after the hot stamping.

In the present disclosure, the method may further include a step ofplating the surface of the steel plate with aluminum (Al)-silicon (Al)for improving corrosion resistance.

Advantageous Effects

According to the present disclosure, it is possible to minimize thevariation in properties of the steel material in the hot-stampingprocess, increase the strength and toughness of the steel material byreducing the martensite packet size, and increase the elongation of thesteel material.

Therefore, the hot-stamped part manufactured by the method according tothe present disclosure exhibits a tensile strength (TS) of 559 to 605MPa, a yield strength (YP) of 360 to 461 MPa, and elongation (EL) of28.5 to 32.7%, is easily processed into a complex shape, and is alsosuitable for use as a shock absorbing element for an automobile crashenergy absorbing member due to its excellent crash absorptionperformance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a process flowchart showing a method for manufacturing ahot-stamped part according to an embodiment of the present disclosure.

FIG. 2 is a process flowchart showing a step of preparing a blank forhot stamping in the method for manufacturing a hot-stamped partaccording to the embodiment of the present disclosure shown in FIG. 1.

FIG. 3 shows process time-dependent changes in the microstructures of ahot-stamped part of a comparative example, and FIG. 4 shows processtime-dependent changes in the microstructure of a hot-stamped part of anexample.

FIGS. 5 to 7 show the surface structures of an example of the presentdisclosure and comparative examples depending on the time oftransferring to a hot-stamping mold.

FIG. 8 shows the surface structure of an example of the presentdisclosure depending on the cooling rate of a hot-stamping mold.

FIG. 9 shows the change in the structure of a steel material by additionof boron to the steel material according to one embodiment of thepresent disclosure.

FIG. 10 is a graph showing the change in elongation in one example ofthe present disclosure and a comparative example as a function of thecontent of manganese.

DETAILED DESCRIPTION Mode for Disclosure

Hereinafter, the present disclosure will be described in detail withreference to the accompanying drawings so that those skilled in the artmay easily carry out the present disclosure. The present disclosure maybe embodied in various different forms and is not limited to theembodiments described in the present specification. Like referencenumerals designate like or similar components throughout the presentspecification. In addition, detailed description of known functions andconfigurations that may unnecessarily obscure the subject matter of thepresent disclosure will be omitted.

A B-pillar, an important component for an automobile crash energyabsorbing member, has a structure in which steel materials havingdifferent strengths are connected to an upper crash support element anda lower shock absorbing element, respectively, and is manufactured bywelding the two steel materials to each other, followed by molding. TheTWB method, which is mainly used in this manufacturing process, refersto a series of processes of manufacturing a part by cutting two steelplates having different thicknesses, strengths and properties into arequired shape, and welding the cut steel plates, followed by pressmolding. The TWB method is capable of welding two steel materials havingdifferent thicknesses to each other so that each part may have arequired property. The crash support element at the upper portion of theB-pillar is made of, for example, an ultrahigh-strength steel materialcorresponding to a class of 120 to 150K, and a member having good shockabsorption performance is connected, by the TWB method, to the lower endof the B-pillar, on which stress is concentrated, thereby improving theability of the B-pillar to absorb shock when an automobile crash occurs.The steel material that is used in the impact absorption element of theB-pillar is generally referred to as a steel for TWB.

Currently, a steel for TWB is being developed as steel having a 70Kclass tensile strength together with a final ferrite-martensite dualphase through a hot-stamping process after hot-rolling and cold-rollingprocesses. In order to form the B-pillar, the 70K class steel for TWBand, for example, a 150K class steel are welded to each other by the TWBmethod, and then hot-stamped.

However, the existing 150K class steel does not undergo changes inproperties thereof during the hot-stamping process, because it obtains a100% martensite structure through the hot-stamping process. However, the70K class steel for TWB has a disadvantage in that the propertiesthereof change rapidly depending on various parameters of thehot-stamping process, for example, the transfer time taken for the steelto be transferred to a hot-stamping mold after heating the steel, or thecooling rate of the blank or the mold. Accordingly, when the 70K classsteel for TWB is welded with the 150K class steel to form a joined steelmaterial and hot stamping is performed on the joined steel material, itis very difficult to control the process parameters, and hence thevariation in properties of the hot-stamped part occurs, and thus thehot-stamped part is not suitable for an automobile crash energyabsorbing member. In order to overcome this problem, in the presentdisclosure, the variation in properties of the steel material isminimized within the range of hot-stamping process parameters throughthe control and precipitation of the components of the steel material.

Steel Material for TWB

One aspect of the present disclosure relates to a steel material for TWBthat undergoes a hot stamping process. In one embodiment, the steelmaterial for TWB according to one aspect of the present disclosureincludes 0.04 to 0.06 wt % of carbon (C), more than 0 wt % and not morethan 0.03 wt % of silicon (Si), 1.2 to 1.5 wt % of manganese (Mn), morethan 0 wt % and not more than 0.018 wt % of phosphorus (P), more than 0wt % and not more than 0.003 wt % of sulfur (S), 0.01 to 0.10 wt % oftitanium (Ti), 0.01 to 0.10 wt % of niobium (Nb), and the balance ofiron (Fe) and inevitable impurities.

The steel material for TWB finally has a tensile strength (TS) of 559 to605 MPa, a yield strength (YS) of 360 to 390 MPa, and an elongation (EL)of 20% or greater, and has a dual-phase structure of ferrite andmartensite, after hot stamping.

In addition, the steel material for TWB may further contain an aluminum(Al)-silicon (Si) plating layer for improving corrosion resistance onthe surface thereof.

The steel material for TWB according to the present disclosure mayreliably exhibit a 55K class tensile strength after being subjected tohot stamping in a state in which it is joined with a 150K class steelmaterial.

Thus, the steel material for TWB may have a greater shock absorptionrate than a conventional 70K class steel material in a part state inwhich it is joined with a 150K class steel material.

Hereinafter, the functions and contents of the components contained inthe steel material for TWB according to the present disclosure will bedescribed.

Carbon (C)

Carbon (C) is a major element that determines the strength and hardnessof the steel material, and is included to ensure the tensile strength ofthe steel material after the hot-stamping (hot-pressing) process. In oneembodiment, carbon (C) is preferably contained in an amount of 0.04 to0.06 wt % based on the total weight of the steel material for TWB. Whencarbon (C) is contained in an amount of less than 0.04 wt %, it may bedifficult to achieve the mechanical strength of the present disclosure,and when carbon (C) is contained in an amount of more than 0.06 wt %,the toughness of the steel material may be reduced.

Manganese (Mn)

Manganese (Mn) is included for the purpose of increasing hardenabilityand strength during heat treatment. Manganese (Mn) is preferablycontained in an amount of 1.2 to 1.5 wt % based on the total weight ofthe steel material for TWB according to the present disclosure. When thecontent of manganese (Mn) is less than 1.2 wt %, the effect of refininggrains is insufficient. On the other hand, when the content of manganese(Mn) is more than 1.5 wt %, a problem arises in that the toughness ofthe steel is degraded due to the occurrence of central segregation andthis content is disadvantageous in terms of the production cost.

Titanium (Ti)

Titanium (Ti) is included for the purpose of increasing strength andtoughness by reducing the martensite packet size. In addition, titanium(Ti) contributes to improving the elongation of the steel by stablyensuring a ferrite region. Titanium (Ti) is preferably contained in anamount of 0.01 to 0.10 wt % based on the total weight of the steelmaterial for TWB according to the present disclosure. When the contentof titanium (Ti) is less than 0.01 wt %, the effect of refining grainsis insufficient. On the other hand, when the content of titanium (Ti) ismore than 0.10 wt %, it may result in a decrease in toughness.

Niobium (Nb)

Niobium (Nb) is contained for the purpose of increasing strength andtoughness by reducing the martensite packet size. In addition, niobium(Nb) contributes to improving the elongation of the steel material bystably ensuring a ferrite region. In one embodiment, niobium (Nb) iscontained in an amount of 0.01 to 0.10 wt % based on the total weight ofthe steel material for TWB according to the present disclosure. Whenniobium (Nb) is contained in an amount of less than 0.01 wt %, theeffect of refining grains of the steel material in the hot-rolling andcold-rolling processes may be insignificant, and when niobium (Nb) iscontained in an amount of more than 0.10 wt %, it may form coarseprecipitates in the steel making process and degrade the elongation ofthe steel material, and may be disadvantageous in terms of theproduction cost.

Silicon (Si)

Silicon (Si) contributes to improving the strength and elongation of thesteel material. However, when silicon (Si) is contained in an amount ofmore than 0.03 wt % based on the total weight of the steel material forhot stamping according to the present disclosure, it may cause surfacedefects and degrade the plating property of the steel material. Thus, inthe present disclosure, silicon (Si) is preferably contained in anamount of more than 0 wt % and not more than 0.03 wt % based on thetotal weight of the steel material for hot stamping.

Phosphorus (P)

Phosphorus (P) is an element that is easily segregated and degrades thetoughness of the steel material. In one embodiment, phosphorus (P) ispreferably contained in an amount of more than 0 wt % and not more than0.018 wt % based on the total weight of the steel material for hotstamping according to the present disclosure. When phosphorus iscontained in an amount within the above-described range, toughness maybe prevented from being degraded. When phosphorus (P) is contained in anamount of more than 0.018 wt %, it may cause cracks during the processand form an iron phosphide compound, thus degrading the toughness of thesteel material.

Sulfur (S)

Sulfur (S) is an element that degrades workability and physicalproperties. In one embodiment, sulfur (S) may be contained in an amountof more than 0 wt % and not more than 0.003 wt % based on the totalweight of the steel material for hot stamping according to the presentdisclosure. When sulfur (S) is contained in an amount of more than 0.003wt %, it may degrade hot-rolling workability and cause surface defectssuch as cracks by producing macro-inclusions.

Hereinafter, a method of manufacturing a hot-stamped part using thesteel material for TWB according to the present disclosure will bedescribed in detail.

Method for Manufacturing Hot-Stamped Part

Another aspect of the present disclosure relates to a method ofmanufacturing a hot-stamped part using the steel material for TWB thatundergoes the TWB process. FIG. 1 is a process flowchart showing themethod of manufacturing a hot stamping part according to the presentdisclosure, and FIG. 2 is a flowchart specifically showing a step ofpreparing a blank for hot stamping, shown in FIG. 1.

Referring to FIG. 1, the method for manufacturing a hot-stamped partaccording to one embodiment of the present disclosure includes the stepsof: (S110) preparing blanks for hot stamping that are composed of twodifferent kinds of steel materials; (S120) joining the blanks for hotstamping to each other to form a joined steel material; (S130)performing hot stamping on the joined steel material to form a moldedbody; and (S140) cooling the molded body to form a hot-stamped part.

Step (S110) of Preparing Blanks for Hot Stamping

Step (S110) of preparing blanks for hot stamping is a step of cuttingtwo different kinds of steel plates for forming a hot-stamped part intoa desired shape according to the intended use, for example, respectivelyforming a first blank which is to be used as a shock absorbing elementand a second blank which is to be used as a crash support element inorder to form an automobile B-pillar.

The first blank is a portion that becomes a shock absorbing element forthe B-pillar after hot stamping, and has a suitable strength to protecta driver upon an automobile crash, and also preferably has an elongationcapable of protecting the driver by absorbing shock when an automobilecrash occurs. According to a preferred embodiment of the presentdisclosure, the first blank is composed of a steel material having atensile strength (TS) of 559 to 605 MPa, a yield strength (YS) of 360 to390 MPa and an elongation (EL) of 20% or greater, and having adual-phase structure of ferrite and martensite, after hot stamping.

The second blank is a portion that becomes a crash support element forthe B-pillar after hot stamping, and is composed of anultrahigh-strength steel material having a tensile strength of 1,200 to1,500 MPa after hot stamping, for example, in order to protect a driverby securing the survival space for the driver when an automobile crashoccurs.

As shown in FIG. 2, the process of forming the first blank may include ahot-rolling step (S210), a cooling/coiling step (S220), a cold-rollingstep (S230), and an annealing heat treatment step (S240).

In the method for manufacturing a hot-stamped part according to thepresent disclosure, the steel slab in a semi-finished product state,which is to be formed into the first blank in the process of forming thefirst blank, includes 0.04 to 0.06 wt % of carbon (C), more than 0 wt %and not more than 0.03 wt % of silicon (Si), 1.2 to 1.5 wt % ofmanganese (Mn), more than 0 wt % and not more than 0.018 wt % ofphosphorus (P), more than 0 wt % and not more than 0.003 wt % of sulfur(S), 0.01 to 0.10 wt % of titanium (Ti), 0.01 to 0.10 wt % of niobium(Nb), and the balance of iron (Fe) and inevitable impurities.

In the steel slab reheating step, the steel slab obtained through acontinuous casting process is reheated at a slab reheating temperature(SRT) of 1,200° C. to 1,250° C., whereby components segregated duringthe casting are re-dissolved into solid solution. If the slab reheatingtemperature (SRT) is lower than 1,200° C., a problem arises in thatcomponents segregated during the casting are not sufficientlyre-dissolved into solid solution, making it difficult to achieve asignificant effect of homogenizing the alloying elements. The slabreheating temperature (SRT) is more preferred for homogenization of thealloying elements as it is higher, but if the slab reheating temperatureis higher than 1,250° C., the austenite grain size may increase, makingit difficult to ensure strength, and baking hardenability and anti-agingproperties may also decrease, and the product cost of the steel platemay merely increase due to an excessive heating process.

In the hot-rolling step (S210), the reheated steel slab isfinish-hot-rolled at a finishing delivery temperature (FDT) of 860 to920° C.

When the finishing delivery temperature (FDT) is excessively low (e.g.,lower than 860° C.), problems arise in that the mixed grain structure bydual-phase region rolling occurs, making it difficult to ensure theworkability of the steel plate, and the workability is reduced due tothe non-uniformity of microstructures. In addition, a rapid phase changecauses the problem of mass flow in hot rolling. The finishing deliverytemperature (FDT) is also favorable for homogenization of the alloyingelements as it is higher, like the SRT, and is determined depending onthe SRT and the number of passes. However, when the finishing deliverytemperature (FDT) is higher than 920° C., austenite grains arecoarsened, resulting in decreases in baking hardenability and anti-agingproperties.

In the cooling/coiling step (S220), the hot-rolled plate is cooled to acoiling temperature (CT) of 620 to 660° C. and coiled. The coilingtemperature affects the re-distribution of carbon (C), and when thecoiling temperature is lower than 620° C., strength is advantageouslyensured, but a problem arises in that ductility decreases rapidly. Onthe other hand, when the coiling temperature is higher than 660° C.,problems arise in that deterioration in formability or strength occursdue to abnormal grain growth or excessive grain growth.

In the cold-rolling step (S230), the coiled steel plate is uncoiled,pickled, and then cold-rolled. At this time, the pickling is performedfor the purpose of removing scales from the coiled plate, that is, thehot-rolled coil produced through the hot-rolling process.

The cold rolling is preferably performed by cold-rolling the pickledplate at a cold-rolling reduction ratio of 60 to 80%. When thecold-rolling reduction ratio is less than 60%, the effect of deformingthe hot-rolled structure is insignificant. On the other hand, when thecold-rolling reduction ratio is more than 80%, problems may arise inthat the cost required for cold rolling increases, the drawability ofthe steel plate decreases, and cracks occur on the edge of the steelplate, resulting in fracture of the steel plate.

The annealing heat treatment step (S240) is a step of subjecting thecold-rolled steel plate material to annealing heat treatment. In oneembodiment, the annealing heat treatment step includes a step of heatingthe cold-rolled steel plate and cooling the heated cold-rolled steelplate at a cooling rate of 20 to 50° C./s. In one embodiment, thecold-rolled steel plate may be heated at a temperature of 700 to 900° C.during the annealing heat treatment. When the cold-rolled steel plate isheated at a temperature within the above-described range, the processefficiency and the strength and formability of the steel material mayall be excellent.

When the cold-rolled steel plate is cooled at a cooling rate of lessthan 20° C./s, the productivity of the steel material may be reduced,and when the cold-rolled steel plate is cooled at a cooling rate of morethan 50° C./s, it may be difficult to ensure the uniform microstructureof the steel material. For example, the cold-rolled steel plate may becooled at a cooling rate of 30 to 40° C./s.

Meanwhile, in the hot stamping step (S130) of FIG. 1, described below,the joined steel material, which is a molding target, is softened byheating at high temperature, and press-molded, followed by cooling.Thus, since the joined steel material is softened by heating at hightemperature, it may be easily press-molded and the mechanical strengthof the steel material is increased by quenching by cooling after themolding. However, since the steel material is heated at a hightemperature of 800° C. or above, iron (Fe) on the surface of the steelmaterial is oxidized to form oxides (scales). For this reason, in oneembodiment of the present disclosure, a certain coating may be formed onthe cold-rolled steel plate after the annealing heat treatment.Specifically, an aluminum (Al)-based metal coating, which has a highermelting point than an organic coating or a zinc (Zn)-based metalcoating, is formed. For example, aluminum (Al)-silicon (Si)-basedplating is performed. The cold-rolled steel plate plated with aluminum(A)-silicon (Si) is prevented from corrosion, and the formation ofscales on the hot surface of the steel plate during transfer to thepress mold is prevented.

Aluminum (Al)-silicon (Si) plating on the steel plate may be performedby a well-known method. One example is a method of coating the steelplate with aluminum (Al)-silicon (Si) by diffusion. In this method, thesteel plate is placed in a heating furnace which may be heated to adiffusion/coating temperature, and then the surface of the steel plateheated to the diffusion/coating temperature is diffusion-coated withaluminum (Al)-silicon (Si). Another method of plating the steel platewith aluminum (Al)-silicon (Si) may be performed by immersing the steelplate in a plating bath, performing aluminum (Al)-silicon (Si) platingon the immersed steel plate, and then performing alloying heat treatmenton the steel plate and cooling the alloying heat-treated steel plate.

Through this plating, an aluminum (Al)-silicon (Si) plating layer isformed on the surface of the steel plate. This plating layer may serveto prevent an oxide scale layer from being formed during ahigh-temperature heat treatment process to be described below.

Meanwhile, the second blank may be formed by performing a hot-rollingstep, a cooling/coiling step, a cold rolling step and an annealing heattreatment step.

In the method for manufacturing a hot-stamped part according to thepresent disclosure, a steel slab in a semi-finished product state, whichis to be formed into the second blank in the process of forming thesecond blank, may include, by wt %, 0.20 to 0.50% carbon (C), 0.05 to1.00% silicon (Si), 0.10 to 2.50% manganese (Mn), more than 0% and notmore than 0.015% phosphorus (P), more than 0% and not more than 0.005%sulfur (S), 0.05 to 1.00% chromium (Cr), 0.001 to 0.009% boron (B), 0.01to 0.09% titanium (Ti), and the balance of iron (Fe) and inevitableimpurities.

In one embodiment, the hot-rolling step may include the steps of:reheating the steel slab at a temperature of 1,200° C. to 1,250° C.;finish-rolling the reheated slab at a temperature of 900° C. to 950° C.;and cooling the hot-rolled steel plate to a temperature of 680° C. to800° C., followed by coiling. Then, the cold-rolling step may include astep of pickling the coiled steel plate, followed by cold rolling. Next,the annealing heat treatment step may include a step of annealing thecold-rolled steel plate at a temperature of 740° C. to 820° C. The platematerial subjected to the annealing heat treatment may be cooled to roomtemperature at a cooling rate of 5 to 50° C./sec, for example.

Step (S120) of Forming Joined Steel Material

The different kinds of first and second blanks are joined to each otherby the TWB process to form a joined steel material. In one embodiment,the first and second blanks may be disposed in such a manner than thefirst blank becomes a shock absorbing element at the lower end of aB-pillar and the second blank becomes a crash support element at theupper portion thereof. Then, the first and second blanks may be weldedto each other, for example, by a butt welding method using a laser.

Hot-Stamping Step (S130)

The joined steel material is heated in a heating furnace at atemperature of about 850 to 950° C. For example, the heating may beperformed at a temperature of 930° C. for about 5 minutes. Next, theheated joined steel material is transferred to a press mold. At thistime, it may take a transfer time of about 9 to 11 seconds. After thejoined steel material is molded into a final part shape in the pressmold for hot stamping, the molded body is cooled rapidly at a coolingrate of about 30 to 120° C./sec to form a final product.

Although not shown in the drawing, the press mold may include therein acooling channel through which a refrigerant circulates. Circulation ofthe refrigerant supplied through the cooling channel can quench theheated blank. At this time, in order to maintain the desired shape whilepreventing the joined steel material from springing back, quenching maybe performed under pressure in a state in which the press mold isclosed.

According to the hot-stamped part manufactured through theabove-described processes (S110 to S130), it is possible to compensatefor strength and secure elongation as a result of maximizingprecipitation instead of reducing the fraction of martensite by limitingthe content of carbon (C) contained in the steel material portioncorresponding to the first blank, which is used in the shock absorbingelement, to 0.04 to 0.06 wt %. In addition, as a result of propertylimiting the contents of titanium (Ti) and niobium (Nb), it is possibleto reduce the martensite packet size, thus increasing the strength andtoughness of the hot-stamped part. Moreover, by stably securing theferrite region, it is possible to increase the elongation and minimizethe variation in properties of the hot-stamped part, which depends onhot-stamping process parameters. In addition, even when expensivehardenable elements such as molybdenum (Mo) are not added, a superior55K class hot-stamped part may be manufactured, which may exhibit shockabsorption performance due to high elongation thereof.

Therefore, the hot-stamped part manufactured by the method according tothe present disclosure includes, as a shock absorbing element, the steelmaterial having a dual-phase structure of ferrite and martensite whilesatisfying a tensile strength (TS) of 550 MPa or greater, a yieldstrength (YP) of 300 MPa or greater and an elongation (EL) of 20% orgreater, and thus is easily processed into a complex shape and alsosuitable for use as an automobile crash energy absorbing member due toexcellent crash absorption performance thereof. In addition, the crashsupport element of the hot-stamped part may maintain a high tensilestrength (TS) of 1200 to 1500 MPa.

Hereinafter, the configuration and effects of the present disclosurewill be described in more detail with reference to preferredembodiments. However, these embodiments are presented as preferredexamples of the present disclosure and cannot be construed as limitingthe present disclosure in any way. In addition, contents that are notdisclosed herein can be sufficiently and technically understood by anyperson skilled in the art, and thus the description thereof is omitted.

1^(st) Embodiment

1. Preparation of Specimens

A steel slab containing the components shown in Table 1 below and thebalance of iron (Fe) and other inevitable impurities was reheated at aslab reheating temperature of 1,200° C., hot-rolled at a finishingdelivery temperature of 900° C., and then cooled and coiled at a coilingtemperature of 640° C., thereby producing a hot-rolled coil. Thehot-rolled coil was uncoiled, and then cold-rolled to produce acold-rolled steel plate. The cold-rolled steel plate was subjected toannealing heat treatment by heating to a temperature of 810° C. and thencooling at a cooling rate of 33° C./s, thereby producing a steelmaterial of Example 1.

In addition, steel materials of Comparative Examples 1 and 2 wereproduced in the same manner as Example 1 above, except that steel slabscontaining the components shown in Table 1 and the balance of iron (Fe)and other inevitable impurities were applied. In the case of the steelmaterial specimen of Comparative Example 2, titanium (Ti) and molybdenum(Mo) were added in amounts different from those of the steel materialspecimen of Comparative Example 1.

TABLE 1 C Mn Nb Ti Mo Target tensile strength Classification (wt %) (wt%) (wt %) (wt %) (wt %) after hot stamping Example 1 0.05 1.40 0.06 0.07— 550 MPa or more Comparative 0.08 1.60 0.05 0.07 — 500 to 700 MPaExample 1 Comparative 0.08 1.80 0.05  0.065 0.20 650 to 750 MPa Example2

Next, first blanks composed of the steel materials of Example 1 andComparative Examples 1 and 2 were prepared. In addition, a second blankcomposed of a steel material having a tensile strength of 1500 MPa wasseparately prepared. The second blank may include, by wt %, 0.20 to0.50% carbon, 0.05 to 1.00% silicon (Si), 0.10 to 2.50% manganese (Mn),more than 0% and not more than 0.015% phosphorus (P), more than 0% andnot more than 0.005% sulfur (S), 0.05 to 1.00% chromium (Cr), 0.001 to0.009% boron (B), 0.01 to 0.09% titanium (Ti), and the balance of iron(Fe) and inevitable impurities.

Each of the first blanks was laser-welded to the second blank, therebyproducing joined steel materials according to Example 1 and ComparativeExamples 1 and 2. Each of the joined steel materials was heated at atemperature of 930° C. for 5 minutes, and then each of the heated joinedsteel materials was transferred to a hot press mold within a transfertime of about 10 seconds and hot-press-molded to produce molded bodies.Each of the molded bodies was cooled at a cooling rate of 100° C./s,thereby manufacturing final hot-stamped parts.

2. Evaluation of Mechanical Properties

Of the produced molded bodies, tensile strength (MPa), yield strength(MPa) and elongation (%) were measured for the steel material portioncorresponding to each of Example 1 and Comparative Examples 1 and 2. Theresults are shown in Table 2 below.

TABLE 2 Tensile strength Yield strength Elongation Classification (TS)(MPa) (YS) (MPa) (EL) (%) Target value 550 MPa or more 300 MPa or more20% or more Example 1 580 370 26.0 Comparative 570 390 13.2 Example 1Comparative 730 530 12.3 Example 2

Referring to the results in Table 2 above, the steel material portion ofExample 1 exhibited better elongation than the steel materials ofComparative Examples 1 and 2 within the ranges of tensile strength andyield strength required for a crash absorbing material. In particular,in the case of elongation, the steel material of the Example of thepresent disclosure exhibited an elongation of 20% or more, suggestingthat it has an excellent ability to absorb shock when an automobilecrash occurs, and thus has excellent performance as a shock absorbingmaterial.

FIG. 3 is a graph showing process time-dependent changes in themicrostructures of the molded bodies corresponding to the steelmaterials of Comparative Examples 1 and 2, and FIG. 4 is a graph showingprocess time-dependent changes in the microstructure of the molded bodycorresponding to the steel material of Example 1.

During the production of the molded bodies of Example 1 and ComparativeExamples 1 and 2, in order to examine transfer time-dependent changes inthe microstructures of the steel materials when transferring the joinedsteel materials to the hot press, measurement was performed at transfertimes of 7 sec, 9 sec, 11 sec and 13 sec. Table 3 below shows thedistribution ranges of tensile strength, yield strength and elongationwithin the transfer time range.

TABLE 3 Tensile strength Yield strength Elongation Classification (TS)(MPa) (YS) (MPa) (EL) (%) Example 1 559 to 605 360 to 390 26.0 to 32.7Comparative 529 to 619 354 to 435  7.0 to 19.4 Example 1 Comparative 685to 773 466 to 601  8.5 to 16.1 Example 2

Referring to Table 3 above, it can be seen that the variations intensile strength, yield strength and elongation within the transfer timerange were greater in the steel materials of Comparative Examples 1 and2 than in Example 1.

Referring to FIG. 3, it can be seen that the steel materials ofComparative Examples 1 and 2 underwent rapid changes in the fractions ofmartensite and ferrite depending on the transfer time or the blank ormold cooling rate after heating for hot stamping. That is, as shown inFIG. 3, it can be seen that the time-axis spacing (reference numeral“A”) between a ferrite transformation curve 320 and a bainitetransformation curve 320 of the steel material is narrow, suggestingthat the properties of the steel material can change rapidly as atemperature curve 330 of the steel material moves left or right on thetime-axis depending on the process parameters. Since it is not easy tocontrol the process conditions of the hot-stamping process, it can beseen that when the fractional variation in microstructural phasesbetween different portions of the molded body occurs as described above,the molded body is not suitable for use as an automobile crash energyabsorbing member.

On the contrary, referring to FIG. 4, it can be seen that a ferritetransformation curve 410 of the steel material was significantly shiftedleft on the time-axis compared to that in FIG. 3. Thus, the spacing(reference number “B”) along the time-axis between the ferritetransformation curve 410 and a bainite transformation curve 420 is wide,and hence even when a time-dependent temperature curve 430 of the steelmaterial is shifted left or right due to process parameters, it shiftsbetween the two transformation curves 410 and 420, and thus thevariation in properties of the hot-stamped part can be minimized. Thatis, the structure of the steel material of Example 1 after hot stampingmay be a dual-phase structure of ferrite and martensite. This resultsfrom limiting the content of carbon (C) and controlling the contents ofniobium (Nb) and titanium (Ti), which makes it possible to stably ensurethe desired properties of the hot-stamped part.

FIG. 5 shows the surface structure of the molded body corresponding tothe steel material of Example 1 of the present disclosure depending onthe time of transfer to the hot-stamping mold; FIG. 6 shows the surfacestructure of the molded body corresponding to the steel material ofComparative Example 1 depending on the time of transfer to thehot-stamping mold; and FIG. 6 shows the surface structure of the moldedbody corresponding to the steel material of Comparative Example 2depending on the time of transfer to the hot-stamping mold.

Referring to FIGS. 5 to 7, the steel material of Example 1 of thepresent disclosure had ferrite and martensite microstructures, and asshown in Table 3, showed a relatively small variation in tensilestrength, yield strength and elongation properties within the time rangeof transfer to the mold. However, it was observed that within the timerange of transfer to the mold, Comparative Example 1 did not ensurerelatively stable ferrite and martensite microstructures, andComparative Example 2 did not ensure relatively stable ferrite andlow-temperature phase microstructures. The low-temperature phase wasobserved as a microstructure of martensite and bainite. In addition, itcould be seen that Comparative Examples 1 and 2 showed a great variationin tensile strength, yield strength and elongation properties dependingon the time range of transfer to the mold as compared to the steelmaterial of Example 1, as shown in Table 3 above.

FIG. 8 shows the surface structure of the molded body corresponding tothe steel material of Example 1 of the present disclosure depending onthe cooling rate of the hot-stamping mold. Specifically, FIG. 8 showsthe surface structures at cooling rates of 30° C./s, 60° C./s and 120°C./s, respectively. As shown in FIG. 8, the fractions of microstructuresdepending on the cooling rate were almost constant, and the variationsin tensile strength, yield strength and elongation depending on thecooling rate were also not great.

Taken the above results together, it can be seen that when titanium (Ti)and niobium (Nb) are added to ensure a ferrite region and increasehardenability in order to prevent the variation in properties of themolded body, which occurs depending on process parameters, such as thetime of transfer to the mold and the cooling rate, which are difficultto control, and when the fraction of martensite is reduced by reducingthe amount of carbon (C) added, the steel material according to theExample of the present disclosure can stably ensure microstructureswithin the range of the hot-stamping process parameters (the time oftransfer to the hot press mold and the cooling rate), and thus canminimize the variation in properties between different portions of themolded body. In addition, it can be seen that even when expensivemolybdenum (Mo) is added, the steel material of the Example of thepresent disclosure has better toughness than the steel materials of theComparative Examples, and thus has excellent economic efficiency.

2^(nd) Embodiment

1. Preparation of Specimens

A steel material of an Example, which has the alloy composition ofExample 1 shown in Table 1 of the 1^(st) embodiment, was prepared bysequentially performing the hot rolling, cold rolling and annealing heattreatment processes as described in the 1^(st) embodiment. In addition,steel materials of a plurality of Comparative Examples, which have analloy composition including more than 0 wt % and not more than 0.0020 wt% of boron added to the alloy composition of Example 1 shown in Table 1of the 1^(st) embodiment, were prepared by performing the processesdescribed in the 1^(st) embodiment under the same process conditions.

Each of blanks, composed of the steel materials of the Example and theplurality of Comparative Examples, was laser-welded to a bank composedof a 150K class steel material, thereby producing joined steel materialsaccording to the Example and the plurality of Comparative Examples,respectively. On the joined steel materials, the hot press process ofthe 1^(st) embodiment was performed, thereby manufacturing finalhot-stamped parts.

2. Evaluation of Mechanical Properties

Of the produced molded bodies, the fraction of bainite produced in thesteel material by addition of boron was measured for the steel materialportion corresponding to each of the Example and the plurality of theComparative Examples.

FIG. 9 is a graph showing the change in structure by the addition ofboron into the steel material according to one embodiment of the presentdisclosure. Referring to FIG. 9, in the steel material of the Example,to which no boron was added, no bainite was observed. However, in thesteel materials of the plurality of Comparative Examples, the fractionof bainite tended to increase as the content of boron increased. Thatis, as shown in FIG. 3, it can be expected that as boron is added to thesteel material, a section in which the temperature curve 330 of thesteel material passes through the bainite transformation curve 320 willincrease in the cooling process. However, as shown in FIG. 4, in thecase of the steel material of the Example, the temperature curve 330 ofthe steel material may not meet the bainite transformation curve 320 inthe cooling process. Thus, the fraction corresponding to the bainitestructure produced in the steel materials of the plurality ofComparative Examples may be included in the fraction of the ferritestructure in the steel material of the Example. Therefore, the steelmaterial of the Example may have better ductility than the steelmaterials of the plurality of Comparative Examples.

3^(rd) Embodiment

1. Preparation of Specimens

A plurality of steel slabs, each including the components shown in Table4 below and the balance of iron (Fe) and other inevitable impurities,were reheated at a slab reheating temperature of 1,200° C., hot-rolledat a finishing delivery temperature of 900° C., and then cooled andcoiled at a coiling temperature of 640° C., thereby producing hot-rolledcoils. The hot-rolled coils were uncoiled, and then cold-rolled, therebyproducing cold-rolled steel plate materials. Next, the cold-rolled platematerials were subjected to annealing heat treatment by heating to atemperature of 810° C. and then cooling at a cooling rate of 33° C./s,thereby producing steel materials of Examples 2 to 5 and ComparativeExamples 3 to 7.

TABLE 4 Classification C (wt %) Mn (wt %) Ti (wt %) Nb (wt %) Example 20.05 1.2 0.07 0.06 Example 3 1.3 Example 4 1.4 Example 5 1.5 Comparative1.6 Example 3 Comparative 1.7 Example 4 Comparative 1.8 Example 5Comparative 1.9 Example 6 Comparative 2.0 Example 7

Then, each of blanks, composed of the steel plate materials of Examples2 to 5 and Comparative Examples 3 to 7, was laser-welded to a blankcomposed of a 150K class steel material, thereby producing joined steelmaterials according to Examples 2 to 5 and Comparative Examples 3 to 7,respectively. Each of the joined steel materials was heated at atemperature of 930° C. for 5 minutes, and then each of the heated joinedsteel materials was transferred to a hot press mold within a transfertime of about 10 seconds and hot-press-molded, thereby producing moldedbodies. The molded bodies were cooled at a cooling rate of 100° C./s,thereby manufacturing final hot-stamped parts.

2. Evaluation of Mechanical Properties

Of the produced molded bodies, the elongation depending on the amount ofmanganese added was measured for the steel material portioncorresponding to each of Examples 2 to 5 and Comparative Examples 3 to7. For measurement of the elongation, 10 specimens for each of Examples2 to 5 and Comparative Examples 3 to 7 were produced, and thenmeasurement of the elongation was performed through a room-temperaturetensile test.

FIG. 10 is a graph showing the changes in elongation of one Example ofthe present disclosure and one Comparative Example as a function of thecontent of manganese. Table 5 below shows the average elongation (%) andstandard deviation of the 10 specimens produced for each of Examples 2to 5 and Comparative Examples 3 to 7.

TABLE 5 Average elongation Standard Classification (%) deviation Example2 28 1.2 Example 3 27 1.6 Example 4 26 1.3 Example 5 23 2.5 ComparativeExample 3 19 2.7 Comparative Example 4 18 3.1 Comparative Example 5 173.4 Comparative Example 6 16 3.6 Comparative Example 7 15 3.9

Referring to Table 5 above and FIG. 10, the steel materials of Examples2 to 5 had better average elongation than the steel materials ofComparative Examples 3 to 7. In addition, the standard deviation ofelongation of the steel materials of Examples 2 to 5 of the presentdisclosure is measured to be lower than those of the steel materials ofComparative Examples 3 to 7. That is, in the case of ComparativeExamples 3 to 7, in which the content of manganese in the steel materialwas 1.6 wt % or more, it is easy to ensure the strength due to anincrease in solid-solution enhancement caused by manganese, but there isa risk that the elongation will decrease and the standard deviation ofthe elongation will increase. In contrast, in the case of Examples 2 to5, after hot stamping, the elongation increases while the standarddeviation of the elongation relatively decreases, and thus theperformance of the part can be stabilized.

4^(th) Embodiment

1. Preparation of Specimens

A plurality of steel slabs, including the alloy compositions of Examples2 to 5 in Table 4 above and the balance of iron (Fe) and otherinevitable impurities, respectively, were reheated at a slab reheatingtemperature of 1,200° C., hot-rolled at a finishing delivery temperatureof 900° C., and then cooled and coiled at a coiling temperature of 640°C., thereby producing hot-rolled coils. The hot-rolled coils wereuncoiled, and then cold-rolled, thereby producing cold-rolled steelplates. The cold-rolled steel plate materials were subjected toannealing heat treatment by heating to a temperature of 810° C. and thencooling at a cooling rate of 33° C./s, thereby producing steel materialsof Examples 2 to 5.

Then, each of blanks, composed of the steel materials of Examples 2 to5, was laser-welded to a blank composed of a steel material having atensile strength of 1500 MPa, thereby producing joined steel materialsaccording to Examples 2 to 5. Each of the joined steel materials washeated at 930° C. for 5 minutes, and then each of the heated joinedsteel materials were transferred to a hot press mold within a transfertime of about 10 seconds and hot-press-molded, thereby producing moldedbodies. The molded bodies were cooled at a cooling rate of 75° C./s,thereby manufacturing final hot-stamped parts.

2. Observation of Microstructures

For the molded bodies, the area fractions of microstructures in thesteel material portions of Examples 2 to 5 were measured. Themeasurement was performed with a known ASTM E562-11 systematic manualpoint count method. The results of measurement of the area fractions areshown in Table 6 below.

TABLE 6 Manganese Ferrite area Martensite area content (wt %) fraction(%) fraction (%) Example 2 1.2 88 to 98 2 to 12 Example 3 1.3 87 to 97 3to 13 Example 4 1.4 88 to 97 3 to 12 Example 5 1.5 87 to 98 2 to 12

Referring to Table 6 above, it was confirmed that the steel materialportions of Examples 2 to 5 showed no great variation depending on thecontent of manganese. Within 1.2 to 1.5 wt %, which is the manganesecontent range of the present disclosure, the steel material portions ofExamples 2 to 5 were observed to be composed of microstructures withferrite having an area fraction of 87 to 98% and martensite having anarea fraction of 2 to 13%.

5^(th) Embodiment

1. Preparation of Specimens

A plurality of steel slabs, each including 0.05 wt % of carbon, 1.4 wt %of manganese, 0.07 wt % of titanium, 0.06 wt % of niobium and thebalance of iron (Fe) and other inevitable impurities, were reheated at aslab reheating temperature of 1,200° C., hot-rolled at a finishingdelivery temperature of 900° C., and then cooled and coiled at a coilingtemperature of 640° C., thereby producing hot-rolled coils. Thehot-rolled coils were uncoiled, and then cold-rolled, thereby producingcold-rolled steel plates. Then, the cold-rolled steel plates weresubjected to annealing heat treatment by heating to a temperature of810° C. and then cooling at a cooling rate of 33° C./s, therebyproducing steel materials.

Each of blanks, composed of the steel materials subjected to theannealing heat treatment, was laser-welded to a blank composed of asteel material having a tensile strength of 1500 MPa, thereby producingjoined steel materials. The joined steel materials were heated at atemperature of 930° C. for 5 minutes, and then each of the heated joinedsteel materials was transferred to a hot press mold within a transfertime of about 10 seconds and hot-press-molded, thereby producing moldedbodies. Then, the molded bodies were cooled at cooling rates of 34°C./s, 63° C./s, 94° C./s and 115° C./s, respectively, therebymanufacturing hot-stamped parts including the steel materials ofExamples 6 to 9.

2. Observation of Microstructures

For the manufactured hot-stamped parts, the area fractions ofmicrostructures in the steel material portions of Examples 6 to 9 weremeasured. The measurement was performed with a known ASTM E562-11systematic manual point count method. The results of measurement of thearea fractions are shown in Table 7 below.

TABLE 7 Cooling rate Ferrite area Martensite area (° C./s) fraction (%)fraction (%) Example 6 34 90 to 98 2 to 10 Example 7 63 88 to 97 3 to 12Example 8 94 88 to 96 4 to 12 Example 9 115 83 to 95 5 to 17

Referring to Table 7 above, it was confirmed that the steel materialportions of Examples 6 to 9 showed no great variation depending on thecooling rate. Within the cooling rate range of 34 to 115° C./s, thesteel material portions of Examples 6 to 9 were observed to be composedof microstructures with ferrite having an area fraction of 83 to 98% andmartensite having an area fraction of 2 to 17%.

Although the present disclosure has been described in detail withreference to the embodiments, various modifications or alternations canbe made by those skilled in the art. These modifications or alternationsmay be considered to fall within the present disclosure withoutdeparting from the scope of the present disclosure. Therefore, the scopeof the present disclosure should be defined by the following claims.

1.-3. (canceled)
 4. A method for manufacturing a hot-stamped part,comprising the steps of: (a) preparing a first blank using a steel slabcomprising 0.04 to 0.06 wt % of carbon (C), 1.2 to 1.5 wt % of manganese(Mn), 0.01 to 0.10 wt % of titanium (Ti), 0.01 to 0.10 wt % of niobium(Nb), and the balance of iron (Fe) and inevitable impurities, and asecond blank obtained by cutting a steel plate provided separately fromthe first blank; (b) welding the first and second blanks to each otherby a tailor-welded blank process, thereby forming a joined steelmaterial; (c) hot-stamping the joined steel material in a press mold,thereby forming a molded body; and (d) cooling the molded body, therebyforming a hot-stamped part.
 5. The method of claim 4, wherein step (a)comprises: (a-1) finish-hot-rolling the steel slab at a finishingdelivery temperature (FDT) of 860° C. to 920° C.; (a-2) cooling thehot-rolled steel plate to a coiling temperature (CT) of 620° C. to 660°C., followed by coiling; (a-3) uncoiling the coiled steel plate,followed by cold rolling; and (a-4) subjecting the cold-rolled steelplate to annealing heat treatment.
 6. The method of claim 4, wherein thesecond blank has a tensile strength of 1,200 to 1,500 MPa after hotstamping.
 7. The method of claim 4, wherein step (c) comprises: (c1)heating the joined steel material at a temperature of 850° C. to 950°C.; and (c2) transferring the heated joined steel material to the pressmold within a transfer time of 9 to 11 seconds.
 8. The method of claim7, wherein the cooling of the molded body in step (d) is performed at arate of 30 to 120° C./s.
 9. The method of claim 4, wherein the firstblank functions as a shock absorbing element for an automotive B-pillar,and the second blank functions as a crash support element for theautomotive B-pillar.
 10. The method of claim 9, wherein the first blankhas a tensile strength (TS) of 550 MPa or greater, a yield strength (YS)of 300 MPa or greater, and an elongation (EL) of 20% or greater, and hasa dual-phase structure of ferrite and martensite, after the hotstamping.
 11. The method of claim 5, further comprising, after step(a-4), a step of plating a surface of the steel plate with aluminum(Al)-silicon (Al) for improving corrosion resistance.